1. Fair enough, my mistake.
2. Of course, but when you want to use the Pi specific hardware, such as the GPU and camera interface the desktop packages get pulled in.
3. The BTT Pi v1.2 accepts 12-24V via screw terminals, and 5V via a USB connector, while retaining the Pi form-factor. The discontinued x86 based Rock Pi X accepted 9-20V, and 5V via a USB connector, while retaining the Pi form-factor. It also incorporated an on board RTC, though this was powered through an external battery. The Banana Pi uses a standard barrel jack for its 5V DC input, while retaining the Pi form-factor. The Odroid N2+, while being larger (and more expensive) than a Pi accepts 12-18V on a standard DC barrel jack, which makes it suitable for in-vehicle use. It also incorporates an RTC. There are other examples. My point is that it's always been a nuisance that you have to go and buy an expensive Micro or C USB cable only to cut and solder it to your power source to power a Pi - that or connect to the 5V pins on the GPIO header, which prevents a hat being fitted. I sometimes replace the 5V & GND pins with longer angled ones which stick out the back of the PCB to get around this.
4. Sigh. Ok, let's do this. Most computers do not have the kind of hardware interfaces that are typically found on microcontrollers, such as GPIO, i2c, SPI, DSI, etc - these days most of them don't even have RS232 ports. Most computers also consume several orders of magnitude more power than a typical microcontroller, making them unsuitable for battery powered applications and requiring complex cooling set-ups for embedded applications. The Pi has all the interfaces you're used to finding on a microcontroller, while drawing a relatively small amount of power, making battery powered and embedded solutions easy to develop. This is what I mean when I say it sits (or used to sit) somewhere between a microcontroller and a full blown Linux system. I know that you know that I know that you knew that this is what I meant.